Modules that return an error during provisioning should still be cleaned
up so that they don't leak any resources they may have allocated before
the error occurred. Cleanup should be able to run even if Provision does
not complete fully.
Use piles from which to draw config values.
Module values can return their name, so now we can do two-way mapping
from value to name and name to value; whereas before we could only map
name to value. This was problematic with the Caddyfile adapter since
it receives values and needs to know the name to put in the config.
This makes it faster and easier to detect broken configurations, but
is a slight performance hit on config loads since we have to re-encode
the decoded struct back into JSON without the module name's key
* set automatic https error type for cert-magic failures
* add state to onload and unload methods
* update reverse proxy to use Provision() and Cleanup()
